DISCOVERY 



317 



Now every integer when divided by four gives a 

 remainder of o, i, 2, or 3. In the first case speculation 

 says the body is composed entirely of heUum, in the 

 second of helium and an atom of hydrogen, in the 

 third of hehum and two atoms of hydrogen, and in the 

 last of helium and eitlier three atoms of hydrogen or 

 something else. But, of course, suggestive as these 

 speculations are, experiment and that alone can say 

 what the constitutions of the different elements really 

 are. Already in one or two instances, where speculation 

 seems clearly right, experiment has shown it is not right. 

 Literary critics of undoubted learning and insight 

 sometimes ascribe the authorship of an anoniiTnous 

 work, on internal evidence, to a certain man, and find 

 later when the name of the real author comes out that 

 their theory of the authorship, though clever, was 

 quite wTong. This is an analogy from the world of 

 letters. 



But although details are at present meagre, the 

 general idea that helium is a constituent of most of the 

 chemical elements is considered to be broadly correct. 

 This view receives encouragement from the astronomi- 

 cal facts that the hottest stars contain the light ele- 

 ments, hydrogen and helium, and, as the star cools, 

 elements of greater atomic weight gradually make their 

 appearance vmtil, in a cold planet like our own, they 

 exist in great variety. This suggests the formation of 

 the heavy and complex elements out of the light and 

 simple ones, of which the chief is helium. 



The liquefaction and solidifying of helium have led 

 to colder temperatures than have ever before been 

 attained. The element does not become liquid until 

 4^°, or solid until 2J^, above the absolute zero of 

 temperature. Before 1908 helium was the only gas 

 that had resisted liquefaction. The main difficulty 

 was to get the gas down to a temperature low enough 

 for the ordinary process of compression and expansion, 

 by which a gas is progressively cooled to its liquefaction 

 point, successfully to begin. After several workers 

 had given the problem up a Dutch experimenter, 

 Kammerlingh Onnes, succeeded. Helium was found 

 to boil at less than 4^° Centigrade above the absolute 

 zero of temperature. The same worker, by allowing 

 the liquid to evaporate quickly, succeeded in obtaining 

 the substance sohd — a marvellous achievement. It is 

 •wonderful to think that man has approached within 

 about two degrees of the very coldest temperature 

 that can exist anywhere in the universe I 



VI 



It was the war that gave a practical use to helium. 

 It is, on the whole, the best gas for filling balloons and 



airships. In two respects hydrogen, which has hitherto 

 been used for this purpose, has decided advantages : 

 it is the lightest gas that is known and therefore the 

 most buoyant, and it is ver\' cheap and easily prepared 

 in quantity ; but, if safety be considered, it cannot be 

 compared for a moment with helium. Many serious 

 accidents happen to balloons and airships: bullets in 

 war time and accidental fires in peace which set the 

 hj'drogen on fire. Helium, when it is pure, is perfectly 

 safe in this respect. It will neither catch fire nor 

 explode. The engines of the airship, indeed, may be 

 placed inside the envelope containing the helium. 

 Nor is the greater density of helium the serious factor 

 which at first it may appear to be. Helium is the second 

 lightest known gas, and its density is nearly twice 

 that of hydrogen. Now the lifting power of a balloon 

 or of an airship is proportional to the difference between 

 the densities of the gas in the envelope, and of the air 

 displaced by it. For an approximate calculation the 

 densities of air, helium, and hydrogen may be taken as 

 14, 2, and I respectively. Thus the lifting power of 

 helium is proportional to 14 — 2, i.e. 12, and that of. 

 hydrogen to 14 — i, i.e. 13. Their lifting powers are 

 therefore as 12 to 13, or, expressed otherwise, that of 

 helium is 92 per cent, of that of hydrogen. This, of 

 course, is not a serious difference, and may be somewhat 

 reduced by heating the hehum electrically, thereby 

 decreasing its density and so increasing its buoyancy. 



The grave difficulty in substituting helium for 

 hydrogen lies in the trouble and expense of getting an 

 adequate supply of the former. The airship R38 had 

 gas-bags which held about 2,750,000 cubic feet, while 

 helium is apparently a rare element, widely distributed 

 perhaps, but always in small quantities. In the 

 atmosphere near the earth only there is, for example, 

 I part by volume of it in 185,000 parts of air. 

 (Higher up, however, it has been calculated, the amount 

 is greater, and sixty miles up it is said to be nearly 

 I part in 200. Other theorisers claim that the atmo- 

 sphere at this height consists almost entirely of helium. 

 But how are you going to bring it to earth ?) Again, 

 in the gases from mineral waters the amount of 

 helium is rarely as high as i per cent., though one 

 spring in France contains more than 5 per cent. About 

 li is the highest percentage that has been found in 

 natural gas. 



These results show that, unless the sources are very 

 abundant, and the methods of separation cheap, 

 helium must be a costly substance. An investigation 

 of the helium sources throughout the Empire, and the 

 best methods of separating it, was begun in 1915. The 

 natural gas of Canada (and later of U.S.A.) was found 

 to be the best source ; the best method of separation 

 to be fractional distillation. Plants were erected which 

 produced 50,000 cubic feet per day at a cost of about 



